The Bulls have a set guard rotation and no room at center. They've got Tyrus at PF, but otherwise minutes are likely available to be had there. The Bulls also have three wings/guards who suffered through various injuries last season (though Salmons played through his), and any time a wing is nicked up there will be plenty of minutes at SF available.
Given that there are potentially minutes available either position, the question remains where will Johnson play and where should he play?
The Bulls hope for versatility with Johnson beating PFs off the dribble while overpowering SFs in the post. Johnson's shown all the skills to do that at in college. Now, there's always a chance with any prospect (even the guys picked top five) that they merely stink and can't do anything, but let's assume that talent isn't an overall problem. We're looking merely at fit.
The range on his jumper combined with his weight and low center of gravity seem to make him a solid PF prospect. He should set a solid pick and be dangerous hitting the 17 footer or rolling to the basket and play effectively in a pick and roll game.
However, his ball handling won't be maximized at the PF position in the offense we saw last season. Johnson would excel taking opposing PFs off the dribble from the perimeter, but the Bulls didn't leave the PF in that position frequently. Maybe Johnson's ability is high enough for Del Negro to add wrinkles that call for the PF to play out on the perimeter more in isolation.
As a SF, Johnson has the ball handling to play the position, and his style fits the offense where he's required to create on his own from the perimeter. The downside is his shooting range won't space the floor properly for Rose, and his defenders may cheat off him if he's behind the three point line in the base set. Johnson wasn't a great college three point shooter and that shot becomes the mid range game in the NBA.
The one large disadvantage of the SF/PF tweener is that they typically need to master two entirely separate positions in the offense. Players are frequently grouped as wings and bigs with the SF/PF tweener needing to understand the ins and outs of both groups. That creates additional problems whenever there is a coaching change as well. It may partially explain why Tyrus has often looked lost.
The movement patterns for a PF and SF are entirely different in the offense while the pattern for a SG/SF or PF/Care virtually interchangable. Johnson needs to master both patterns to have a successful year because as the eigth man when four are wings and three are bigs, he'll probably split his minutes amonst both groups.
Unless there is an injury pushing Johhnson into a more defined role, I look for him to struggle quite a bit in his rookie year, especially early, when his minutes and responsibilities shift greatly from day to day. It's a tough adjustment for everyone, and it's made tougher by his circumstances.